CA1158333A - Absolute dc system for a laser inspection system - Google Patents
Absolute dc system for a laser inspection systemInfo
- Publication number
- CA1158333A CA1158333A CA000371985A CA371985A CA1158333A CA 1158333 A CA1158333 A CA 1158333A CA 000371985 A CA000371985 A CA 000371985A CA 371985 A CA371985 A CA 371985A CA 1158333 A CA1158333 A CA 1158333A
- Authority
- CA
- Canada
- Prior art keywords
- laser beam
- receiver
- photomultiplier tube
- radiation
- inspection system
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N21/88—Investigating the presence of flaws or contamination
- G01N21/93—Detection standards; Calibrating baseline adjustment, drift correction
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N21/88—Investigating the presence of flaws or contamination
- G01N21/89—Investigating the presence of flaws or contamination in moving material, e.g. running paper or textiles
Abstract
ABSTRACT OF THE DISCLOSURE
A laser inspection system is provided having an absolute dc system in which the power gain of the system is stable and adjustable in order that the resultant signals which are generated from the system may be utilized to identify flaws as distinguished from flaws which may be identified simply by a change in signal relative to the background value. In order to stabilize the power gain of the system and provide an absolute reference dc level or back-ground, a sample of the radiation from the laser beam which is utilized to scan the material being inspected is applied to the photomultiplier tube for generating a reference pedestal signal which is dependent on the intensity of the laser beam and is free of the influence of the characteristics of the material or product being inspected. The reference pedestal signal is applied to a comparator which develops a control signal which is applied in a single feedback loop which includes in effect the laser source and the light collec-tion system or receiver within the loop. This loop includes the photo-multiplier tube and its high voltage power supply to which the control signal is applied for stabilizing the gain of the system. Accordingly, if the sensitivity of the receiver varies, or the photomultiplier drifts or if drift occurs in the power supply of the photomultiplier tube, or if the laser beam intensity drifts, the feedback loop will detect these changes and compensate for them, produce a stable and adjustable power gain for the system. Accord-ingly, changes in response of the photomultiplier tube with respect to a pro-duct being examined will be due to product type changes, for example, products which differ in diffusivity, reflectivity, etc., and will not be due to changes in the system parameters such as sensitivity, temperature, etc.
A laser inspection system is provided having an absolute dc system in which the power gain of the system is stable and adjustable in order that the resultant signals which are generated from the system may be utilized to identify flaws as distinguished from flaws which may be identified simply by a change in signal relative to the background value. In order to stabilize the power gain of the system and provide an absolute reference dc level or back-ground, a sample of the radiation from the laser beam which is utilized to scan the material being inspected is applied to the photomultiplier tube for generating a reference pedestal signal which is dependent on the intensity of the laser beam and is free of the influence of the characteristics of the material or product being inspected. The reference pedestal signal is applied to a comparator which develops a control signal which is applied in a single feedback loop which includes in effect the laser source and the light collec-tion system or receiver within the loop. This loop includes the photo-multiplier tube and its high voltage power supply to which the control signal is applied for stabilizing the gain of the system. Accordingly, if the sensitivity of the receiver varies, or the photomultiplier drifts or if drift occurs in the power supply of the photomultiplier tube, or if the laser beam intensity drifts, the feedback loop will detect these changes and compensate for them, produce a stable and adjustable power gain for the system. Accord-ingly, changes in response of the photomultiplier tube with respect to a pro-duct being examined will be due to product type changes, for example, products which differ in diffusivity, reflectivity, etc., and will not be due to changes in the system parameters such as sensitivity, temperature, etc.
Description
3~3 This invention relates to a stabilized absolute direct current laser inspection system, and more particularly to such a system with a single feed-back loop which can compensate for changes in the laser light source and the light collec~ion system such that -the flaw signals produced by the system are influenced by the product inspected and not the system which is doing the inspecting.
In light inspection systems, radiation, for example, in the fo-rm of a laser beam, is scanned successively across the surface of material being examined and the intensity of the beam ei-ther reflectedJ transmitted or scattered from the material is detected by a receiver which includes a photo-multiplier tube. The signal generated by the photomultiplier tube is called the product pedestal signal which varies in accordance with the characteristics of the material being examined thereby offering a method of determining whether flaws exist in the material or not by varia~ions in the signal detected. The product pedestal signal also includes a certain dc level which will be present based on the intensity of the laser beam, system sensi-tivity, etc., regardless of the type of materials being examined or whether flaws exist in the material or not. This signal may be referred to as the back-ground signal or the dc level of the system which in effect does not depend on product changes but rather on inspection system variations. Por many applica-tions, a relative change in the signal level is all that is required for determining flaws while in certain types of web products absolute changes are required for the purposes of identifying flaws. In the latter application, the background signal level of the system must be stabilized in order that changes in the product pedestal level will be reflected only by changes in -the characteristics of the material being examined.
Accordingly, it is an object of th:is invention to provide a new and ~ :~5~3~3 novel stabilized absolute direct current laser inspection system which is simple in implementati.on and capable of measuring absolute changes in prodwct characteristics.
Another object of this invention is to provide a new and improved laser scalmer flaw detection sys-tem in which the signal utilized for stabilization is free of external environmenlal considerations.
According to a broad aspect of the invention there is provided a stabilized absolute direct current system for controlling the sensitivity of a light inspection system having a laser generating a laser beam and a scanner for scanning said laser beam over a surface of a moving material being inspected thereby to detect flaws in said material, a receiver for collecting laser beam radiation emanating from said material~ said receiver having a photomultiplier tube for detecting and generating signals in accordance with the intensity of radiation applied thereto, the improvement comprising: means for applying a sample of radiation from said laser beam to said photo-multi.plier tube for generating a reference pedestal signal dependent on the intensity of said laser beam which is free of the influence of the character-istics of the material, comparator means having a reference potential and said reference pedestal signal applied thereto for deriving a control signal, a power supply for said photomultiplier tube, a feedback circuit for coupling said control signal to said power supply for automatically regulating the power gain to said photomultiplier tube to a constant value thereby providing a stabilized dc system.
In carrying out this invention in one illustrative embodiment there-of, a laser inspection system is provided having a scanner for scanning a laser beam over a target being inspected which is a moving web of mater:ial. A
receiver is provided for collecting laser beam radiation emanating from the
In light inspection systems, radiation, for example, in the fo-rm of a laser beam, is scanned successively across the surface of material being examined and the intensity of the beam ei-ther reflectedJ transmitted or scattered from the material is detected by a receiver which includes a photo-multiplier tube. The signal generated by the photomultiplier tube is called the product pedestal signal which varies in accordance with the characteristics of the material being examined thereby offering a method of determining whether flaws exist in the material or not by varia~ions in the signal detected. The product pedestal signal also includes a certain dc level which will be present based on the intensity of the laser beam, system sensi-tivity, etc., regardless of the type of materials being examined or whether flaws exist in the material or not. This signal may be referred to as the back-ground signal or the dc level of the system which in effect does not depend on product changes but rather on inspection system variations. Por many applica-tions, a relative change in the signal level is all that is required for determining flaws while in certain types of web products absolute changes are required for the purposes of identifying flaws. In the latter application, the background signal level of the system must be stabilized in order that changes in the product pedestal level will be reflected only by changes in -the characteristics of the material being examined.
Accordingly, it is an object of th:is invention to provide a new and ~ :~5~3~3 novel stabilized absolute direct current laser inspection system which is simple in implementati.on and capable of measuring absolute changes in prodwct characteristics.
Another object of this invention is to provide a new and improved laser scalmer flaw detection sys-tem in which the signal utilized for stabilization is free of external environmenlal considerations.
According to a broad aspect of the invention there is provided a stabilized absolute direct current system for controlling the sensitivity of a light inspection system having a laser generating a laser beam and a scanner for scanning said laser beam over a surface of a moving material being inspected thereby to detect flaws in said material, a receiver for collecting laser beam radiation emanating from said material~ said receiver having a photomultiplier tube for detecting and generating signals in accordance with the intensity of radiation applied thereto, the improvement comprising: means for applying a sample of radiation from said laser beam to said photo-multi.plier tube for generating a reference pedestal signal dependent on the intensity of said laser beam which is free of the influence of the character-istics of the material, comparator means having a reference potential and said reference pedestal signal applied thereto for deriving a control signal, a power supply for said photomultiplier tube, a feedback circuit for coupling said control signal to said power supply for automatically regulating the power gain to said photomultiplier tube to a constant value thereby providing a stabilized dc system.
In carrying out this invention in one illustrative embodiment there-of, a laser inspection system is provided having a scanner for scanning a laser beam over a target being inspected which is a moving web of mater:ial. A
receiver is provided for collecting laser beam radiation emanating from the
-2 ~ 1~8333 web and the receiver includes a photomultiplier tube for detecting and gene-rating signals in accordance wi~h the intensity of radiation applied thereto.
A sample of the radiation from the laser beam is taken from the scanner and applied directly to the photomultiplier tube for generating a reference pedestal signal which is dependent on the intensity of the laser beam but free of material or product influences. The reference pedes~al signal is compared to a reference signal for deriving a control signal which is applied to a closed loop including the power supply for the photomultiplier tube which regulates the power gain of the high voltage power supply to thereby provide a stabilized absolute dc system.
The sample of radiation in one form may be applied to the photo-multiplier tube by a light pipe~ or in another form by a stabilized sensor which is scanned by the laser beam for converting the light signal of the laser b~am into electrical form which is amplified and applied to a tempera-ture compensated light emitting diode which is in communication with the photomultiplier tube.
Advantageouslyl the positioning of the light pipe or the detector in proximity of the scanner avoids any excess overscan in the system as well as removes the receiving end of the light pipe or the detector from the influ-ences of the external environment.
The invention~ together with further aspects, objects and advantages thereof, will be better understood from the following description taken in connection with the accompanying drawings, in which:
FIGURE 1 is a schematic block diagram of the stabilized absolute direct current laser inspection system embodied in the present invention.
FIGURES 2a and 2b illustrate waveforms which are generated by and useful in the explanation of the operation of FIGURE 1.
FIGURE 3 is a diagrammatic illustration of the use of the present invention when two photomultiplier tubes are utilized in the receiver.
FIGURE 4 is a diagrammatic illustration of the use of a light pipe for the derivation of a reference pedestal in accordance with the present invention.
FIGURE ~ is a diagrammatic illustration of alternative embodiments of the present invention in which the light pipe or photodetector are posi-tioned adjacent the leading and trailing edges of the scanned target area.
In the following description, like elements will bear the same reference numerals. Merely for purposes of illustration a transmissive type system is illustrated in which flaws are detected based on the transmissive properties of the material being examined. However, a reflective type system where the output of ~he system is proportional to the reflectivity of the spot on which the laser beam is impinging may also be utilized in which case the receiver would be positioned above the target. Whether a transmissive or a reflective or scattering mode is utilized will depend upon the application and the type of material which is being examined.
Referring now to FIGURE 1, a scanner referred to generally with the reference character 10 includes a suitable source of radiation, for example~ a light beam 14 generated by a laser 12 which is applied after reflection from folded mirrors 16 and 18 and through spot forming optics 19 to a rotary scanner 20. The rotary scanner 20 is a conventional multi-facet mir-rored surface polygon which is driven by a motor ~not shown~ in the direction of ~he arrow on the drawing. It will be apparent that the laser along with the folded mirrors and the spot forming optics may be positioned externally of the scanner 10. It will also be apparent that different types of scanners may be employed~ for example oscillating mirrors, rotating prisms, etc. The rotary : .
- . ' ' : . :.
.: ~
~ 15~3~1 scanner 20 performs the function of successively scanning the beam 14 across a continuously moving web of material 25 which is moving in a dir~ction orthogonal to the plane of the drawing. The rotary scanner 20 causes the beam 14 to scan across the surface of the material 25 and scanning in the orthogonal direction to create a raster is accomplished automatically by the movement of the web of material 25. Light transmitted through the material 25 is applied to a receiver 30 having a suitable detector therein such as a photomultiplier tube ~PMT) 32 which detects the light applied thereto. Al-though different types of receivers may be utilized, the type illus-trated is a light conducting rod 30 having a diffused stripe 31 on the bottom thereof such that radiation applied through the target 25 from the scanner 10 will be scattered and internally reflected in the rod 30 and applied to the face of the photomultiplier tube 32. Another type of receiver which may be employed is shown and described in United States Patent 3,900,265 issued August 19, 1975, which is assigned to the assignee of the present invention.
As in typical light inspection systems, at any instant of time dur-ing the scan, the detector 32 provides an output signal which is proportional to the transmission of the spot on the material 25 on which the laser beam 14 is impinging. Flaws occurring in the material 25 being examined change the output of the detector 32 due to the transmissive or scattering properties of -the material being examined providing a means for indicating flaws in the material. As is previously pointed out, in a reflective system the receiver 30 would be positioned above the material Z5 to receive reflected radiation scattered or specularly reflected from the surface of the moving web 25.
As a result of the scanned beam 14 passing across the product 25, the light energy collected by the receiver 30 will go from zero when off the product 25 to a finite amount when on the product by reason of the scattering or transmissive properties of the web and to zero again when off the product at the trailing edge which pedestal signal is shown in FIGURE 2a. The electrical signal devel.oped by the detector 32 is a classical rectangular function commonly referred to as a product pedestal signal. As long as the beam power, product equivalent optical density and receiver sensi.tivity remain fixed, the height of the product pedestal signal will remain fixed. If how-ever the beam power or the recei~er sensitivity changes~ the height of the pedes-tal will change, and it will not be known i:E the cause of the change is due to the product or is due to a power change or gain of the system.
In order to assure that the change in signal level of the detector is due solely to the product to thus produce an absolute dc system, the power gain of the system is stabilized by a control loop which includes the light source as well as the ligh~. collection system or receiver within the loop.
Since the two systems parameters most subject to drift with tinne~ temperature and other environmental properties are the laser beam power and the gain of the photomultiplier tube, a way for regulating the consequences of these changes is to couple a portion of the beam energy within the scanner directly to the face of the photomultiplier tube.
FIGURE 1 illustrates one implementation of this in which a stabilized sensor 3~, such as a solar cell or temperature compensated sillcon detector, is positioned within or attached to the scanner 10 for converting the light signal in accordance with the intensi-ty of the laser beam 14 into electrical form. The sensor output signal 34 is applied to an amplifier 36 whose amplitude is adjustable by a potentiometer 38 and the amplified output is applied to drive a light emitting diode 40 which is embedded wi.thin the receiver 30 in proximity to the face of the photomultiplier tube 32. Although it is preferable to embed the light emitting diode 40 close to the face of the ~ ~5~3 PMT, FIGURE 1 illustrates the positioning of the light emitting diode at different positions along the receiver 30, if it is desired to do so. The adjustment of the potentiometer 38 is equivalent to setting different refer-ence levels, and thus controlling the sensitivity of the system which is de-sirable when examing different product types.
In order for the stabilized sensor 34 to function for its intended purpose, it is necessary to sample electronically the signal generated by the detector so that its level can be compared with an adjustable voltage refer-ence. The difference signal thus formed is amplified and applied to an appropriate component within the system to provide a constant power gain. An appropriate element for this purpose is the high voltage power supply associ-ated with the photomultiplier tube 32. The gain of the photomultiplier 32 is directly related to the high voltage applied to the tube. Accordingly, changes in laser beam power, photomultiplier tube sensitivity or other compon-ents within the loop are therefore corrected in the illustrated embodimen-t of FIGURE 1 by a feedback system resul~ing in a fixed system sensitivity capable of measuring absolute changes in product characteristics.
The sensor output 34 is applied via PMT 32 to a sample and hold cir-cuit 42. ~nce per facet timing pulses 44 (l/f) are applied from the rotary scanner 20 to an adjustable one shot multivibrator 46 which samples the output from the sensor 34 from the composite PMT output. This reference sample pedestal 48 is illustrated in FIGURE 2b and is applied from the sample and hold circuit 42 to a differential amplifier 50 having a reference potential 52 applied to the other input thereof. The difference output representing changes in the level of the reference pedestal 48 is applied to the PMT high voltage power supply 54 whose output is coupled to the PMT 32. Accordingly, changes in laser beam power, PMT sensitivity and other components within the ~ ;1 5 ~
loop including the PMT high voltage power supply 54 are therefore compensated via the feedback system which stabilizes the gain of the system.
Placing the sensor 34 within the scanner 10 and coupling its output directly -to the face of the photomultiplier tube 32 eliminates the need for excessive overscan of tlle scanner as well as removes the detector 34 and its associated output from influences of the external enviro~lent.
FIGURE 3 illustrates a receiver 30 which employs two photomultiplier tubes thereby including a photomultiplier tube 66 in addition to the photo-multiplier tube 32. In such an application, it is desirable although not absolutely necessary that a second stabili~ed sensor 60 be employed and be positioned on each side of the procluct corresponding to the leading and trail-ing edges of the moving web 25. As is illustrated in FIGURE 3, the output of the sensor 60 is applied -to an amplifier 62 and to a light emitting diode 64 positioned adjacent the face of the photomultiplier tube 66. In this embodi-ment two control systems of the type illustrated in FIGURE 1, namely two closed loops, one for each photomultiplier tube would be required.
Another embodiment of the present invention is illustrated in FIGURE
4 which in effect is the simplest implementation of coupling a reference source of light directly from the laser beam to the photomultipli.er tube in the form of an optical fiber or light pipe coupled from the edge limits of the scanned interval as illustrated by the light pipe 70 in FIGURE 4. A light pipe would therefore be coupled to the PMT during an overscan and the result-ant pedestal signal 48 as illustrated in FIGURE 2b would be treated in the same manner by the control system as illustrated in FIGURE 1. The amount of light coupled into the light pipe 70 can be controlled by a field stop thus providing a desired adjustmeJIt for referencing different product types and receiver configurations. It will be apparent of course that an additional
A sample of the radiation from the laser beam is taken from the scanner and applied directly to the photomultiplier tube for generating a reference pedestal signal which is dependent on the intensity of the laser beam but free of material or product influences. The reference pedes~al signal is compared to a reference signal for deriving a control signal which is applied to a closed loop including the power supply for the photomultiplier tube which regulates the power gain of the high voltage power supply to thereby provide a stabilized absolute dc system.
The sample of radiation in one form may be applied to the photo-multiplier tube by a light pipe~ or in another form by a stabilized sensor which is scanned by the laser beam for converting the light signal of the laser b~am into electrical form which is amplified and applied to a tempera-ture compensated light emitting diode which is in communication with the photomultiplier tube.
Advantageouslyl the positioning of the light pipe or the detector in proximity of the scanner avoids any excess overscan in the system as well as removes the receiving end of the light pipe or the detector from the influ-ences of the external environment.
The invention~ together with further aspects, objects and advantages thereof, will be better understood from the following description taken in connection with the accompanying drawings, in which:
FIGURE 1 is a schematic block diagram of the stabilized absolute direct current laser inspection system embodied in the present invention.
FIGURES 2a and 2b illustrate waveforms which are generated by and useful in the explanation of the operation of FIGURE 1.
FIGURE 3 is a diagrammatic illustration of the use of the present invention when two photomultiplier tubes are utilized in the receiver.
FIGURE 4 is a diagrammatic illustration of the use of a light pipe for the derivation of a reference pedestal in accordance with the present invention.
FIGURE ~ is a diagrammatic illustration of alternative embodiments of the present invention in which the light pipe or photodetector are posi-tioned adjacent the leading and trailing edges of the scanned target area.
In the following description, like elements will bear the same reference numerals. Merely for purposes of illustration a transmissive type system is illustrated in which flaws are detected based on the transmissive properties of the material being examined. However, a reflective type system where the output of ~he system is proportional to the reflectivity of the spot on which the laser beam is impinging may also be utilized in which case the receiver would be positioned above the target. Whether a transmissive or a reflective or scattering mode is utilized will depend upon the application and the type of material which is being examined.
Referring now to FIGURE 1, a scanner referred to generally with the reference character 10 includes a suitable source of radiation, for example~ a light beam 14 generated by a laser 12 which is applied after reflection from folded mirrors 16 and 18 and through spot forming optics 19 to a rotary scanner 20. The rotary scanner 20 is a conventional multi-facet mir-rored surface polygon which is driven by a motor ~not shown~ in the direction of ~he arrow on the drawing. It will be apparent that the laser along with the folded mirrors and the spot forming optics may be positioned externally of the scanner 10. It will also be apparent that different types of scanners may be employed~ for example oscillating mirrors, rotating prisms, etc. The rotary : .
- . ' ' : . :.
.: ~
~ 15~3~1 scanner 20 performs the function of successively scanning the beam 14 across a continuously moving web of material 25 which is moving in a dir~ction orthogonal to the plane of the drawing. The rotary scanner 20 causes the beam 14 to scan across the surface of the material 25 and scanning in the orthogonal direction to create a raster is accomplished automatically by the movement of the web of material 25. Light transmitted through the material 25 is applied to a receiver 30 having a suitable detector therein such as a photomultiplier tube ~PMT) 32 which detects the light applied thereto. Al-though different types of receivers may be utilized, the type illus-trated is a light conducting rod 30 having a diffused stripe 31 on the bottom thereof such that radiation applied through the target 25 from the scanner 10 will be scattered and internally reflected in the rod 30 and applied to the face of the photomultiplier tube 32. Another type of receiver which may be employed is shown and described in United States Patent 3,900,265 issued August 19, 1975, which is assigned to the assignee of the present invention.
As in typical light inspection systems, at any instant of time dur-ing the scan, the detector 32 provides an output signal which is proportional to the transmission of the spot on the material 25 on which the laser beam 14 is impinging. Flaws occurring in the material 25 being examined change the output of the detector 32 due to the transmissive or scattering properties of -the material being examined providing a means for indicating flaws in the material. As is previously pointed out, in a reflective system the receiver 30 would be positioned above the material Z5 to receive reflected radiation scattered or specularly reflected from the surface of the moving web 25.
As a result of the scanned beam 14 passing across the product 25, the light energy collected by the receiver 30 will go from zero when off the product 25 to a finite amount when on the product by reason of the scattering or transmissive properties of the web and to zero again when off the product at the trailing edge which pedestal signal is shown in FIGURE 2a. The electrical signal devel.oped by the detector 32 is a classical rectangular function commonly referred to as a product pedestal signal. As long as the beam power, product equivalent optical density and receiver sensi.tivity remain fixed, the height of the product pedestal signal will remain fixed. If how-ever the beam power or the recei~er sensitivity changes~ the height of the pedes-tal will change, and it will not be known i:E the cause of the change is due to the product or is due to a power change or gain of the system.
In order to assure that the change in signal level of the detector is due solely to the product to thus produce an absolute dc system, the power gain of the system is stabilized by a control loop which includes the light source as well as the ligh~. collection system or receiver within the loop.
Since the two systems parameters most subject to drift with tinne~ temperature and other environmental properties are the laser beam power and the gain of the photomultiplier tube, a way for regulating the consequences of these changes is to couple a portion of the beam energy within the scanner directly to the face of the photomultiplier tube.
FIGURE 1 illustrates one implementation of this in which a stabilized sensor 3~, such as a solar cell or temperature compensated sillcon detector, is positioned within or attached to the scanner 10 for converting the light signal in accordance with the intensi-ty of the laser beam 14 into electrical form. The sensor output signal 34 is applied to an amplifier 36 whose amplitude is adjustable by a potentiometer 38 and the amplified output is applied to drive a light emitting diode 40 which is embedded wi.thin the receiver 30 in proximity to the face of the photomultiplier tube 32. Although it is preferable to embed the light emitting diode 40 close to the face of the ~ ~5~3 PMT, FIGURE 1 illustrates the positioning of the light emitting diode at different positions along the receiver 30, if it is desired to do so. The adjustment of the potentiometer 38 is equivalent to setting different refer-ence levels, and thus controlling the sensitivity of the system which is de-sirable when examing different product types.
In order for the stabilized sensor 34 to function for its intended purpose, it is necessary to sample electronically the signal generated by the detector so that its level can be compared with an adjustable voltage refer-ence. The difference signal thus formed is amplified and applied to an appropriate component within the system to provide a constant power gain. An appropriate element for this purpose is the high voltage power supply associ-ated with the photomultiplier tube 32. The gain of the photomultiplier 32 is directly related to the high voltage applied to the tube. Accordingly, changes in laser beam power, photomultiplier tube sensitivity or other compon-ents within the loop are therefore corrected in the illustrated embodimen-t of FIGURE 1 by a feedback system resul~ing in a fixed system sensitivity capable of measuring absolute changes in product characteristics.
The sensor output 34 is applied via PMT 32 to a sample and hold cir-cuit 42. ~nce per facet timing pulses 44 (l/f) are applied from the rotary scanner 20 to an adjustable one shot multivibrator 46 which samples the output from the sensor 34 from the composite PMT output. This reference sample pedestal 48 is illustrated in FIGURE 2b and is applied from the sample and hold circuit 42 to a differential amplifier 50 having a reference potential 52 applied to the other input thereof. The difference output representing changes in the level of the reference pedestal 48 is applied to the PMT high voltage power supply 54 whose output is coupled to the PMT 32. Accordingly, changes in laser beam power, PMT sensitivity and other components within the ~ ;1 5 ~
loop including the PMT high voltage power supply 54 are therefore compensated via the feedback system which stabilizes the gain of the system.
Placing the sensor 34 within the scanner 10 and coupling its output directly -to the face of the photomultiplier tube 32 eliminates the need for excessive overscan of tlle scanner as well as removes the detector 34 and its associated output from influences of the external enviro~lent.
FIGURE 3 illustrates a receiver 30 which employs two photomultiplier tubes thereby including a photomultiplier tube 66 in addition to the photo-multiplier tube 32. In such an application, it is desirable although not absolutely necessary that a second stabili~ed sensor 60 be employed and be positioned on each side of the procluct corresponding to the leading and trail-ing edges of the moving web 25. As is illustrated in FIGURE 3, the output of the sensor 60 is applied -to an amplifier 62 and to a light emitting diode 64 positioned adjacent the face of the photomultiplier tube 66. In this embodi-ment two control systems of the type illustrated in FIGURE 1, namely two closed loops, one for each photomultiplier tube would be required.
Another embodiment of the present invention is illustrated in FIGURE
4 which in effect is the simplest implementation of coupling a reference source of light directly from the laser beam to the photomultipli.er tube in the form of an optical fiber or light pipe coupled from the edge limits of the scanned interval as illustrated by the light pipe 70 in FIGURE 4. A light pipe would therefore be coupled to the PMT during an overscan and the result-ant pedestal signal 48 as illustrated in FIGURE 2b would be treated in the same manner by the control system as illustrated in FIGURE 1. The amount of light coupled into the light pipe 70 can be controlled by a field stop thus providing a desired adjustmeJIt for referencing different product types and receiver configurations. It will be apparent of course that an additional
3 3 light pipe can be utilized on the other side of the scanner for providing light pulses in both the leading and trailing edges of the target 25 which can be used with two PMT detectors in accordance with the embodiment of FIGURE 3 or may be used with one as desired.
~ IGUR~ 5 illustrates the positioning of the sensor 60 or the light pipe 70 along the leading and trailing edges of the target 25 which are hit by the overscan of the target. Again the sensitivity of the system is controlled by the closed loop arrangement shown in FIGURE 1. The purpose of this illus-tration is merely to show that the sensors or the light pipes which ever are used may be positioned adjacent the target as distinguished from their posi-tioning in the scanner as illustrated in the previous embodiments. Of course, the benefit of removing the generated reference pedestal from the external environment is somewhat diminished in this embodiment.
Since other changes and modifications varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the examples chosen for purposes of illustration, and covers all changes and modifications which do not constitute a departure from the true spirit and scope of this invention.
~ IGUR~ 5 illustrates the positioning of the sensor 60 or the light pipe 70 along the leading and trailing edges of the target 25 which are hit by the overscan of the target. Again the sensitivity of the system is controlled by the closed loop arrangement shown in FIGURE 1. The purpose of this illus-tration is merely to show that the sensors or the light pipes which ever are used may be positioned adjacent the target as distinguished from their posi-tioning in the scanner as illustrated in the previous embodiments. Of course, the benefit of removing the generated reference pedestal from the external environment is somewhat diminished in this embodiment.
Since other changes and modifications varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the examples chosen for purposes of illustration, and covers all changes and modifications which do not constitute a departure from the true spirit and scope of this invention.
Claims (7)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A stabilized absolute direct current system for controlling the sensitivity of a light inspection system having a laser generating a laser beam and a scanner for scanning said laser beam over a surface of a moving material being inspected thereby to detect flaws in said material, a receiver for collecting laser beam radiation emanating from said material, said receiver having a photomultiplier tube for detecting and generating signals in accordance with the intensity of radiation applied thereto, the improvement comprising: means for applying a sample of radiation from said laser beam to said photomultiplier tube for generating a reference pedestal signal dependent on the intensity of said laser beam which is free of the influence of the characteristics of the material, comparator means having a reference potential and said reference pedestal signal applied thereto for deriving a control signal, a power supply for said photomultiplier tube, a feedback circuit for couyling said control signal to said power supply for automatically regulating the power gain to said photomultiplier tube to a constant value thereby pro-viding a stabilized dc system.
2. The stabilized absolute direct current light inspection system set forth in claim 1 wherein said means for applying a sample of radiation from said laser beam to said photomultiplier tube comprises a light pipe.
3. The stabilized absolute direct current light inspection system set forth in claim 2 in which said light pipe is positioned at one end thereof in said scanner and on the other end thereof in said receiver for coupling radia-tion on the overscan of said scanner directly to said receiver.
4. The stabilized absolute direct current light inspection system set forth in claim 2 in which said light pipe is positioned on one end thereof adjacent an edge of said moving web and on the other end thereof in said receiver for coupling radiation on the overscan of said scanner directly to said receiver.
5. The stabilized absolute direct current light inspection system set forth in claim 1 wherein said means for applying a sample of radiation from said laser beam to said photomultiplier tube comprises a sensor for detecting light from said laser beam, an amplifier coupled to said sensor for amplifying the signal generated by said sensor and a light emitting diode positioned in said receiver and coupled to the output of said amplifier.
6. The stabilized absolute direct current light inspection system set forth in claim 5 in which said sensor is positioned in said scanner.
7. The stabilized absolute direct current light inspection system set forth in claim 6 in which said sensor is positioned adjacent an edge of said moving material.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/128,286 US4297587A (en) | 1980-03-07 | 1980-03-07 | Absolute DC system for a laser inspection system |
| US128,286 | 1980-03-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1158333A true CA1158333A (en) | 1983-12-06 |
Family
ID=22434570
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000371985A Expired CA1158333A (en) | 1980-03-07 | 1981-02-27 | Absolute dc system for a laser inspection system |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4297587A (en) |
| JP (1) | JPS56140244A (en) |
| CA (1) | CA1158333A (en) |
| DE (1) | DE3108344A1 (en) |
| FR (1) | FR2477712B1 (en) |
| IT (1) | IT1194760B (en) |
| SE (1) | SE8101446L (en) |
Families Citing this family (15)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4447150A (en) * | 1981-02-27 | 1984-05-08 | Bentley Laboratories | Apparatus and method for measuring blood oxygen saturation |
| JPS5892935A (en) * | 1981-11-30 | 1983-06-02 | Mitsubishi Electric Corp | Tester for surface defect |
| US4538915A (en) * | 1981-12-23 | 1985-09-03 | E. I. Du Pont De Nemours And Company | Web inspection system having a product characteristic signal normalizing network |
| JPS6010174A (en) * | 1983-06-29 | 1985-01-19 | Fuji Photo Film Co Ltd | Screening method of gene by auto radiography |
| EP0148288B1 (en) * | 1983-12-22 | 1990-02-28 | S.P.A. Leglertex | Apparatus for detecting anomalies in corduroy preparation |
| US4701985A (en) * | 1983-12-22 | 1987-10-27 | Leglertex S.R.L. | Apparatus for detecting anomalies in corduroy preparation |
| DE3418283A1 (en) * | 1984-05-17 | 1985-12-12 | Schott Glaswerke, 6500 Mainz | METHOD FOR DETECTING DEFECTS IN TRANSPARENT MATERIALS |
| NL9100248A (en) * | 1991-02-13 | 1992-09-01 | Philips & Du Pont Optical | DEVICE FOR MEASURING THE REFLECTION AND / OR TRANSMISSION OF AN ARTICLE. |
| JPH06207910A (en) * | 1993-01-11 | 1994-07-26 | Fuji Photo Film Co Ltd | Surface inspection apparatus |
| USH1616H (en) * | 1994-05-31 | 1996-12-03 | Minnesota Mining And Manufacturing Company | Web inspection system having enhanced video signal preprocessing |
| US6526077B1 (en) * | 2000-05-25 | 2003-02-25 | Nelson Tabirian | Line-scan laser beam profiler |
| US9747776B2 (en) | 2012-06-08 | 2017-08-29 | Neilsen-Kuljian, Inc. | Electrical sensor with configurable settings and a display |
| US9664709B2 (en) | 2012-06-08 | 2017-05-30 | Neilsen-Kuljian, Inc. | Electrical sensor with configurable settings |
| US9766273B2 (en) * | 2012-08-21 | 2017-09-19 | Neilsen-Kuljian, Inc. | Self-powered current sensor with a linear adjustment |
| US9478402B2 (en) | 2013-04-01 | 2016-10-25 | Kla-Tencor Corporation | Photomultiplier tube, image sensor, and an inspection system using a PMT or image sensor |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3690774A (en) * | 1971-02-26 | 1972-09-12 | Dow Chemical Co | Continuous optical quality control monitor |
| US3866054A (en) * | 1973-09-28 | 1975-02-11 | Du Pont | Defect size discriminator circuit for web inspection system |
| US3900265A (en) * | 1974-03-08 | 1975-08-19 | Intec Corp | Laser scanner flaw detection system |
| US3958127A (en) * | 1974-08-09 | 1976-05-18 | E. I. Du Pont De Nemours And Company | Optical-electrical web inspection system |
-
1980
- 1980-03-07 US US06/128,286 patent/US4297587A/en not_active Expired - Lifetime
-
1981
- 1981-02-27 CA CA000371985A patent/CA1158333A/en not_active Expired
- 1981-03-05 DE DE19813108344 patent/DE3108344A1/en not_active Withdrawn
- 1981-03-05 IT IT20150/81A patent/IT1194760B/en active
- 1981-03-06 FR FR8104562A patent/FR2477712B1/en not_active Expired
- 1981-03-06 JP JP3238381A patent/JPS56140244A/en active Pending
- 1981-03-06 SE SE8101446A patent/SE8101446L/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| IT8120150A0 (en) | 1981-03-05 |
| FR2477712A1 (en) | 1981-09-11 |
| IT1194760B (en) | 1988-09-28 |
| DE3108344A1 (en) | 1981-12-24 |
| JPS56140244A (en) | 1981-11-02 |
| US4297587A (en) | 1981-10-27 |
| SE8101446L (en) | 1981-09-08 |
| FR2477712B1 (en) | 1985-07-05 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA1158333A (en) | Absolute dc system for a laser inspection system | |
| BuSSe et al. | Thermal wave imaging with phase sensitive modulated thermography | |
| US3433052A (en) | Material tester | |
| US3864044A (en) | Method and apparatus for the analysis of a dispersed phase capable of transmitting and focusing light | |
| US5026991A (en) | Gaseous species absorption monitor | |
| US3994590A (en) | Discrete frequency colorimeter | |
| US4356448A (en) | Apparatus for measuring electrical or magnetic fields by absorption spectrum change | |
| EP0360126A3 (en) | Operation method for an optical smoke detector and smoke detector for carrying out the method | |
| RU94028284A (en) | Method for photon scanning tunnel microscopy and photon scanning tunnel microscope | |
| EP0200342A3 (en) | Evaluation of surface and subsurface characteristics of a sample | |
| US6031620A (en) | Gloss sensor resistant to tilting and shifting paper and with improved calibration | |
| KR20060061371A (en) | Gas detection method and gas detector device | |
| JP2711225B2 (en) | Improved data collection method for methods using chemical sensors | |
| KR920700388A (en) | Single beam AC interferometer | |
| US4570074A (en) | Flying spot scanner system | |
| US4921346A (en) | Tunnel current detecting photo-acoustic spectrometer | |
| GB1493967A (en) | Method of and apparatus for measuring the width of an elongated element | |
| US4097751A (en) | Retroreflectance measuring apparatus | |
| US4881818A (en) | Differential imaging device | |
| US6526372B1 (en) | Method and device for determining the dependence of a first measuring quantity on a second measuring quantity | |
| US5008558A (en) | System for detecting minute particles on or above a substrate | |
| US4607955A (en) | Stock consistency transmitter | |
| KR950004361A (en) | Focused ion beam apparatus, focused ion beam observation method and focused ion beam processing method | |
| US20050190365A1 (en) | Method and device for interrogating samples using laser scanning cytometry and other techniques | |
| EP0606849A2 (en) | Surface inspecting apparatus |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |